It has been reported in the prior art that a quartz oscillator may be used to measure the concentration of a substance in a solution. Specifically, it has been reported that the concentration of a substance in a solution may be determined by measuring the change in oscillation frequency due to a change in mass on the surface of the quartz oscillator.
For example, one of the present inventors used the method for assay of a substance in a solution by measuring the change in the oscillator frequency during the direct binding of the substance to the quartz oscillator, and have determined the concentration of silver ion in an aqueous solution by taking advantage of the fact that the silver ion in the solution will electrodeposit on the electrode of the oscillator naturally (e.g., T. Nomura, et al., Anal. Chem. Acta, 169, 257 (1985)).
On the other hand, Muramatsu, et al., used the method for assay of a substance in a solution by binding the substance indirectly to a quartz oscillator, and have assayed IgG in the solution by using the piezoelectric crystal with Protein A immobilized thereon (e.g., H. Muramatsu, et al., Anal. Chem., 59, 2760 (1987)). It was also reported in Japanese Patent Laid Open No. 62-64934 to immobilize an antibody on the surface of the crystal and detect a specific microbe by the antigen-antibody reaction.
On the other hand, an electrode-separated piezoelectric crystal oscillator having its electrode separated from the crystal wafer was reported by W. G. Cady (e.g., W. G. Cady, J. Gen. Appl. Phys., 7, 237 (1936)). What has been reported by W. G. Cady is to compare the oscillating frequency in the case where the electrode is separated from the crystal wafer with the frequency in the unseparated case. However, W. G. Cady has failed to show the capability of assay of a specific substance or its concentration by using the separate electrode-separated piezoelectric crystal oscillator.
The quartz oscillator used in the prior art for measuring a substance is of the type in which a thin film of an electrode is adhered on the crystal wafer. The electrode is applied to the crystal wafer by vacuum evaporation or by an electro-plating process, etc. The electrode layer thus formed will cause unstable oscillation of the oscillator if excessively thick, and will be liable to come off if excessively thin, reducing its practicability. Therefore, the thickness of the electrode layer has to be precisely controlled by troublesome operations. Moreover, it is necessary that a lead wire be attached to the electrode on the crystal wafer in order to apply an electrical voltage to the electrode from an oscillation circuit. This limits the reduction of the size of the electrode. Since the quartz oscillator is used in a solution, on the other hand, the oscillation frequency due to only the binding of a substance to be assayed cannot be accurately measured if the electrode is dissolved or comes off.
Therefore, this makes it necessary to use a noncorrosive precious metal such as gold, platinum, or palladium as the material for the electrode. Thus, the fabrication of the quartz oscillator of the prior art has problems of troublesome processes, high costs, and difficulties in fabricating products of uniform quality and small size. Hence, such oscillators having high production costs cannot be generally used as measurement elements of disposable type, so that it has to be repeatedly used for the measurements. Since, in this case, the troublesome operations are required for rinsing (or reactivating) the oscillator after each measurement, the electrode cannot be avoided from erosion or the like when at the rinsing step, even if it is made of the above-specified noncorrosive material. As a result, the responsiveness of the oscillator is degraded after each measurement, reducing the reproducibility of the measurements. Thus, it is difficult to measure many samples for a short time period, and the reliability of the assay results obtained has a problem.
Since the quartz oscillator of the prior art is united with the electrode, the weight of the oscillator is increased by the electro-deposition of the substance coexisting in the solution on the electrode surface, thus causing a problem of interfering with the accurate measurement of the frequency change.